1. Field of the Invention
This invention relates to a mobile radio communication system such as a portable radio telephone system or a land mobile radio telephone system, and more particularly to a communication system based on a cellular system.
2. Description of the Related Art
FIG. 10 shows a schematic diagram of a conventional portable or land mobile radio telephone system using the cellular system. Such a system is composed of a control station CS connected to a wire telephone network NW, a plurality of base stations connected to the control station CS via wire links (land lines) CL1 to CLn, and a plurality of mobile stations PS1 to PSm. The base stations BS1 to BSn have radio zones E1 to En, called cells, in different areas, respectively. The mobile (radio) stations PS1 to PSm are connected via radio links to the base stations in the radio zones where they are at present. Each base station is then connected to the wire telephone network NW via the control station CS. With this configuration, the mobile station is able to communicate with a telephone set connected to the wire telephone network. When any one of mobile stations PS1 to PSm is moving from one radio zone to another while communicating, the control station CS switches from the current radio link to that of the new radio zone, thereby maintaining the continuity of the communication now in operation.
FIG. 11 is a diagram illustrating the originating sequence of a mobile station in the above system. For example, it is assumed that the hook switch of mobile station PS3 is turned on to originate a call (for a calling request). The mobile station PS3 first selects a radio frequency for control, and generates an originating signal including its identification code (ID code). It then transmits the originating signal to the radio base station BS2 via a control channel of the radio frequency. Receiving the originating signal from mobile station PS3, the base station BS2, which has been monitoring the occurrence of a calling request on the control channel in the waiting state, produces a calling signal including the ID code of mobile station PS3 and its own ID code and transfers it to the control station CS.
The control station CS judges whether or not this calling request is from its own system based on the ID codes contained in the calling signal. If they are ID codes of the system, the station CS sends back to the base station BS2 a calling answer signal ACK including speech channel designation (specifying) information to specify a radio frequency for talking. After the base station BS2 has received the calling answer signal from the control station CS, it generates an originating answer signal including speech channel designation (specifying) information and the ID code of the calling mobile station PS3, and transmits it to the mobile station PS3 via the control channel.
After mobile station PS3 has received the originating answer signal, it attempts to establish a speech channel of the above frequency to the base station BS2, according to the speech channel designation (specifying) information in the answer signal. Once the speech channel has been established, a calling signal is sent from the control station CS to the wire telephone network NW. Receiving this calling signal, the network NW sends a signal requesting the transmission of a dial signal to the calling mobile station PS3 via the control station CS and base station BS2. The mobile station PS3, receiving the dial signal transmission request, indicates that dialing is possible.
The caller acknowledges this indication and dials the called party's number on the dial plate. The mobile station PS3 then transmits to the base station BS2 a dial signal corresponding to the dial number via the radio speech channel. Receiving the dial signal, the base station BS2 transfers this signal to the network NW via the control station CS. The network NW then performs a switching operation to call a desired telephone. Once the called party has answered the call, getting his telephone off the hook, a channel is established between the called party's telephone and the calling mobile station PS3. From this time on, communication is possible through the channel between them.
For portable and land mobile radio telephone systems based on the cellular system, the need for more mobile stations to be served is increasing. To meet this demand, a method has been proposed which transmits the digitized signal by a TDMA (Time Division Multiple Access) system, where not only control signals but also speech signals have been digitized. In the TDMA system, the transmitted signal is constructed in frames for each radio frequency. One frame is then time-divided into, for example, 6 time slots as shown in FIG. 12, each slot being allocated as a communication channel to a separate mobile station. This system can cover nearly six times as many mobile stations as a conventional equivalent.
In today's cellular radio telephone systems, a radio frequency is allocated permanently to each of base stations BS1 to BSn, with the same radio frequency allocated again to another base station separated enough not to introduce interference. This raises the following problem in spite of using the TDMA system.
In a communication system using the TDMA system, one radio frequency offers, for example, 6 communication channels as described earlier. Even when only one channel is used by a base station, however, the corresponding radio frequency is occupied by this low-traffic base station. As a result, for example, even if a base station with higher traffic, adjacent to the low traffic base station, wants to use the idle channels of the radio frequency being used, it is virtually impossible for the higher traffic station to use the idle ones.
A system may be considered which allows the control station to supply a reference synchronizing signal to each base station via a control line to synchronize time slots between base stations. This system, however, requires the laying of a control line from the control station to each base station. Depending on the length of each control line, the delay of the reference synchronizing signal must be compensated for. This not only makes the facilities larger, but also requires more complicated synchronizing control to harmonize the timing of the reference synchronizing signal to each base station. The greater the number of base stations, the more complicated the synchronizing control. The complexity of synchronizing control sometimes makes it impossible to realize such a communication system.